Zoologger: The semaphore ‘worms’ as big as sperm whales

TALK about teamwork. Huge free-floating coalitions of marine invertebrates known as pyrosomes have to coordinate to ensure the colony heads the right way – and light may be their secret to doing so.

The tubular pyrosomes are made up of many clones called zooids. For the colony to feed and move, each zooid sucks in water from outside and blows it down the tube to form a rudimentary jet engine. Shutting off this propulsion system allows it to sink out of harm’s way – but how does the signal pass through the colony when they lack any common nerves to communicate?

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David Bennett at Bangor University, UK, found that when a pyrosome is brushed by an external object, it lights up like a Christmas tree. In unpublished research he suggests it is this signal that ripples through the zooid cells, telling them to cut their engines.

Pyrosomes are rarely seen, though, partly because we’re not looking in the right places, says Mangesh Gauns of India’s National Institute of Oceanography in Goa. He analysed conditions off the coast of India where a swarm was seen and found that pyrosomes need cyanobacteria small enough for the zooids to swallow along with the right mineral balance in the water (Zoological Studies, doi.org/23j). These conditions should be commonplace away from coastal waters, which are dominated by larger plankton that can block the zooids’ filter-feeding system.

This article appeared in print under the headline “Hollow marine monsters may use light signalling to dive”